1. Field of the Invention
This invention relates to a system for exchanging data over a communication channel and more specifically to modems having an improved means for detecting loss of synchronization.
2. Description of the Prior Art
High speed data communication systems make use of an initial training period in which the modems at each end of the communications channel are synchronized. Prior art systems make a decision to retrain because of loss of synchronization between the modems when the output of an integrator located in a modem receiver indicates that the normalized equalizer error is greater than a preset threshold. However such systems have proven to be unreliable indications of loss of synchronism because such channel impairments as noise spikes can move a received constellation point into an area which is a valid area for another point, and thus no error signal is produced for the point which has been incorrectly shifted. Thus a loss of synchronism condition may not be detected if a substantial proportion of the errors in point placement due to it are not detected because these errors placed the points within a valid area for other points.